Lush Antarctic past suggests more monsoons in future

During the ice-free Eocene period 56 to 34 million years ago, Earth was 10 °C warmer than today. “Tropical” animals like crocodiles roamed the polar regions, and Antarctica – now a frigid desert – was once warm and covered with lush vegetation. Identification of these plants suggests that Antarctica had a monsoon, something that only occurs in the tropics these days. It could mean that in a warmer world monsoons spread across the planet.

Frédéric Jacques of the Xishuangbanna Tropical Botanical Garden in Mengla, China and colleagues excavated fossil plants from an Antarctic island. As certain plants only grow in particular climates, their fossils provide evidence that such a climate prevailed in the past.

Jacques’ fossils came from plants that experience seasonal rainfall. He estimates that 60 per cent of Antarctica’s annual rainfall fell in summer during the Eocene, about 6.4 millimetres per day.

Downpour

That suggests there was a monsoon on the continent. Eocene Antarctica looks much the same as monsoon climates of south Asia today, where there is heavy rainfall in summer but the winters are dry.

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Finding a monsoon so close to the poles is unexpected, says Gill Martin of the UK Met Office in Exeter, UK. “It tends to be thought of as a tropical thing.”

Monsoon rains are driven by seasonal changes in wind direction. Moist sea air blows onto the land during summer, while in winter the winds travel in reverse. For this to happen, the land must warm up much more than the ocean during summer. Today, this only happens in the tropics, but Jacques says that monsoons may proliferate in hotter climates. “It can create new monsoon circulations.”

Our climate future

Could the monsoons also spread? It’s unlikely we’ll reach Eocene temperatures unless we burn fossil fuels well into the 2200s. Even then, Antarctica is likely to stay frozen, so an Antarctic monsoon probably isn’t on the cards. But Jacques says monsoons could spring up on other continents.

Martin is unconvinced. She says major weather systems like the jet streams operate close to the poles, and could interfere with any new monsoons.